The present invention relates to an image forming apparatus, such as a copier and printer and, in particular, to an image forming apparatus having a plurality of photosensitive drums for forming an image of developing agents of respective colors on the basis of a color-separated image signal and a conveying belt for supplying a recording sheet (transfer medium) past the respective photosensitive drums.
A four-unit tandem type color copier is known as an image forming apparatus for outputting a color image, the color copier forming toner images of the colors, for example, yellow (Y), magenta (M), cyan (C) and black (B) on the basis of a color-separated image signal and transferring these toner images on a recording sheet through the registration of the toner images on the recording sheet. This type of color copier has four photosensitive drums arranged, side by side, in a manner to be spaced apart a predetermined distance and allows four color toner images to be carried thereon and a conveyer belt for supplying the recording sheet past the four photosensitive drums. The conveying belt is stretched between a pair of mutually spaced-apart rollers and run in an endless fashion. The respective photosensitive drums are driven by corresponding individual motors and one of these rollers is also driven by its own motor.
The respective photosensitive drum and conveying belt a rearranged in a contacting relation and, if there occurs a difference in speed between the photosensitive drum and the conveyor belt, then one of the two is injured. In order to prevent any undesired friction caused by the speed difference, it is necessary to move the respective photosensitive drum and conveyor belt constantly in the same direction and at the same speed. For example, when the photosensitive drum and conveying belt are started, it is necessary to drive the two at the same timing. At the start of both the motors, therefore, a relatively great drive current is required.
In the fourth-unit tandem type color copier having the above-mentioned four photosensitive drums, in particular, there occurs an overlap in overshoot at the starts of the respective motors, thus exerting a greater burden on a power source unit of the copyer.